Photographic study of solid propellants burning in an acceleration environment

Abstract

Previous studies have shown that acceleration forces may substantially increase the burning rate of aluminized composite solid propellants by retaining burning Al agglomerates on the propellant surface. This paper describes detailed observations, made by high-speed color cinematography, of the effect of acceleration on the Al agglomeration and combustion processes occurring on the propellant surface. Six different composite propellants were photographed while burning at pressures of 200, 500, and 1000 psia, and at accelerations up to 100 g in a direction normal (90°) to and into the burning surface. Additional films were taken of one propellant with the acceleration vector oriented at 75° to the burning surface. The films show that Al agglomerates, which under static conditions also form on the burning surface but are quickly entrained by the combustion gases, were held on the surface when the acceleration forces were present, and coalesced into burning globules. The retention of these globules resulted in the formation of pits in the propellant surface because of the increase in burning rate near the globule. The globules increased in size by collecting additional Al agglomerates emerging from the pit walls, and the pits continued to grow due to the increasing heat transfer to the propellant surface beneath the burning globules. When the pits covered a large part of the surface they began to merge, forming broader and shallower pits in which the globules coalesced; this created still larger globules which flattened out in the acceleration field. As combustion proceeded, the pits continued to merge, and the globules to flatten until they covered almost the entire surface. This observed history of pit growth is described in terms of a five-stage combustion model, which provides an explanation for the burning-rate transients under acceleration observed in other studies. All aluminized propellants studied exhibited the same mechanism of pit formation and growth; however, the time required for the transition of the pits through the various stages of their development dependent upon the propellant formulation and upon pressure and acceleration level. A high-burning-rate propellant, containing a Mg-Al alloy, did not exhibit any acceleration sensitivity. Size distributions of the Al agglomerates on the burning surface were measured from individual frames of the films taken of four of the propellants burning at various pressures and accelerations. The mass median diameters of these agglomerates at zero acceleration were widely different for the different propellants, ranging from 130 to 440 μ. The diameter appeared to increase with acceleration and with decreasing pressure level. It was noted that the propellants which formed the larger agglomerates had exhibited greater sensitivity to acceleration in previous experiments. Samples of residual slag and extinguished propellant surfaces were studied, using photographs taken with a scanning electron microscope. The surfaces of the slag particles contain large numbers of small holes, and the interior structure is characterized by tubular voids. The appearance of the AP particles in photomicrographs of extinguished propellants suggests the subsurface evolution of gas and the existence of a liquid layer during combustion.